The protein interactome of the Neuron Specific Gene family (NSG1-3).

The Neuron-Specific Gene (NSG) family members (NSG1-3) play critical and diverse roles in neuronal protein trafficking, but their precise molecular functions remain poorly understood. Here, we employed proximity-labeling proteomics to map the interactomes of each NSG protein. Unlabeled mass spectrometry identified over 1,000 significantly enriched interactors compared with a cytoplasmic control, revealing substantial overlap between NSG1 and NSG2, and a more divergent profile for NSG3. Gene ontology and KEGG pathway analysis confirmed established associations with glutamatergic synapses and endosomal trafficking, while also uncovering unexpected links to presynaptic machinery, inhibitory synapses, and endoplasmic reticulum-associated protein translation, particularly for NSG3. Reciprocal biotinylation patterns and co-immunoprecipitation revealed novel heteromeric complex formation between NSG1 and NSG2, with limited interactions involving NSG3. All three NSGs biotinylated core AMPA receptor subunits and auxiliary proteins, while NSG1 and NSG2 also associated with NMDA receptors, GABA receptor subunits, as well as multiple presynaptic proteins. Moreover, NSG1 and NSG2 specifically biotinylated components of multiunit tethering complexes including neuron-specific retromer, and biotinylated a preponderance of ADAM10 substrates, reinforcing their role in proteolytic processing. Finally, despite the relatively divergent interactomes of NSG1 and NSG2 compared to NSG3, all family members robustly biotinylated amyloid precursor protein (APP), suggesting possible synergistic or competitive interactions that could shape APP proteolytic processing and/or trafficking. Together, these data provide a comprehensive systems-level view of NSG protein interactions, establishing a molecular framework for future investigations into NSG-mediated neural plasticity and disease mechanisms.